Control apparatus



Jan. 19, 1954 Filed March 15, 1951 R. J- EHRET ETI'AL CONTROL APPARATUS 3 Sheets-Shqet l MEASURING msmuuem FURNACE 4 FIG. I s g (5 u'mfij'mq FAST RESET 8 RATE CONTROL Q '5 INSTRUMENT v M suns WIRE RESET AME CONTROL CONTROL AUR lNPUT GYGUNG RESET SIGNAL amuse f CONTROL AMPLIFIER cmcun AMPLIFIER 5 AND RELAYS M I H 9 9191- lo ONTROL FAST RESET cYcLms AND CONTROL SLOW asser cou'rnm.

SIGllAL sum 1. Y

FIG 4 72 K 91s 49 70B [74 q'a i 27 32 o HEAT 1 98 couvune 49.

HEAT 1, COUPLIN 'W-uiFf AUTO.

INVENTORJ'. ROBERT J. EHRET ROGER F.WERNLUND ATTORNEY.

Jan. 19, 1954 R. J. EHRET ETAL CONTROL APPARATUS Filed March 15 1951 I5 Sheets-Sheet 2 HEAT COUPLING FIG. 2

mRw E m N NER R .E o hJw T mm m W Rwu w M W 8 Jan- 19, 1954 R. J. EHRET ETAL CONTROL APPARATUS 3 Sheets-Sheet 3 INVENTORS. ROBERT J. EHRET ROGER F. WERNLUND ATTORNEY.

Y om B iv @e 9 on. K2 5. w 2? 3. Ne v :2: #60.

Filed March 15. l951 Patented Jan. 19, 1954 UNITED STATES 2,666,889 CONTROL APPARATUS Robert J. Ehret, Philadelphia, and Roger Wernlund, Southampton, Pa.,-

assignors to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a, corporation of Delaware Application March 15, 1951, Serial No. 215,846

37 Claims.

1 The present invention relates generally to automatic electric control apparatus, and relates specifically to automatic electric proportioning con-' trol apparatus of the time-proportioning type, wherein a control action is made proportional inmagnitude to the need therefor by adjustment of the ratio of the control on time to off time in-accordance with the demand of the process or other arrangement being controlled.

A general objectof the present invention is to provide improved automatic electric control apparatus of the time-proportioning type, by means of which proportional control of a process or other arrangement is effected in an improved manner by theautomatic adjustment of the ratio of the control on time to the control off time in accordancewith the demand or need for control action of the controlled arrangement.

In the automatic control of processes requiring a proportional type of controlfor proper operation, numerous processes have been encountered which cannot tolerate so-called throttling con-- trol, wherein the instantaneous rate of flow of an agent is made proportional to a measured value of a controlled condition. Examples of such processes are certain fuel-fired ones which cannot tolerate fuel valve throttling in the control of fuel flow rate to maintain a constant temperature.

As an illustration, it is noted, that ,a wide car bottom furnace must befired at maximum rate, when heat is needed, in order that the flame be uniformly distributed so as to produce the same degreeof heat across the charge. ,In such a furnace, if the temperature is controlled by proportional adjustment or throttling of the rate of fuel flow, undesirably uneven heating of the charge occurs.

However, if the temperature of such a furnace is controlled on a time-proportioning basis, uniform heating across the charge results. Such control is effected by operating the fuel supply valve between maximum and minimum open positions, and by proportioning the fuel input to the furnace in pulses, the ratio, during any given period, of "valve maximum open or ,on time to the :totalperiod of time beingrmade proportional to the need for heat as indicated by the measured furnace temperature. By definition, the total timeis equal to the valve maximum open or on. time plusthe valve minimum open or 01f time.

Time proportioning control is also required for the control of the temperature of electric furnaces where it is not feasible to vary the furnace heater energizing current between its maximum and minimum or off values, and Where, therefore, the heating current must be fully on when heat is called for. The control apparatus of the present invention fulfills; this requirement, by propor tioningv or regulatingthe furnace heat input by varyingflthe, ratio of heating currentflonftime to total time in accordance with the furnace load de-- mand.

, Accordingly, it is an object of the present invention to provide control apparatus of the type specified above which is extremely well adapted to effect close, accurate control of the value of a condition by proportioningthe ratio of control on time to total time as required to maintain said value at a predetermined, desired figure, and which eifects such control electronically, with a minimum of control action lag or delay and with a minimum of over-compensation, thereby reducing to a minimum any tendencies .of the apparatus toover-shoot or hunt.

A specific object of the invention is to provide electronic time-proportioning control apparatus of the type specified which includes an electronically operated controllable automatic reset portion capable of providing and maintaining required reset effects in the apparatus, whereby the occurrence of oifset or droop, in the value of the controlled condition from a desired value is prevented, and whereby the magnitude of excursions of the controlled condition from the desired value is kept at anabsoluteminimum atcall times.

Another specific object of the invention is to provide apparatus of the specifiedtype which provides a desirable rate response action in effecting its controlling functions.

A more specific object of the invention is to provide control apparatus of the type just specified wherein the proportional cycling and reset functions are performed by temperature sensitive resistors which areselectively heated by suitable heaters, the latter in turn being controlled by electronic amplifiers whose heater controlling actions are influenced by the valueof the controlled condition and by the resistance values of the temperature sensitive resistors.

Other specific objects of the present invention are to provide control apparatus of the type above specified including means, for adjusting the rate of operation of the automatic reset means over a wide range of operating values, and to provide. in such apparatus approach rate control means for limiting the reset rate to a controlled maximum value in one direction only.

A still morespecific object of the invention is to provide control apparatus as specified above including an adjustable feedback connection for providing accurate control of the cycle time of the apparatus without adversely affecting the sensitivity thereof,

An even more specific object of the present invention is to provide control apparatus of the type last specified wherein the various controls, such as for proportional band, cycle time, and reset rate, act independently of one another, whereby the adjustment of the characteristics of the control apparatus to suit the characteristics of the process to becontrolled is rendered as simple as possible.

Another more specific object of the invention is to provide control apparatus as specified above which includes the necessary circuit components and connections to provide so-called safe-failure of the apparatus upon the development of various component defects therein.

When suitably connected to a process or other arrangement to be controlled, the apparatus of the present invention is operative to effect timeproportioning control actions so as to maintain a selected condition of the process at a predetermined, desired value. When that condition is the temperature within an electric furnace, for example, and when this temperature is exactly at the predetermined or control point value, the load relay of the apparatus will be cycled in such a manner that the furnace heater will be alternately energized and deenergized, the ratio, during each cycle, of heater energized or on time to the total cycle time being that required to maintain the control point temperature within the furnace. (Total time equals on time plus oir time.)

Upon a deviation in the furnace temperature from the control point value, due to a furnace load or similar change, the furnace heater will be energized for a greater or a lesser portion of each cycle, depending upon the direction of the temperature deviation. The magnitude of this corrective effect or change in on total time ratio will be proportional to the extent and rate of the temperature deviation. age rate of heat input to the furnace will be changed in accordance with the rate and extent of the departure of the temperature from the control point.

. Also upon a temperature deviation as just discussed, the reset portion of the apparatus becomes effective to perform its function of producing an effect in the apparatus which causes the ratio of furnace heater on time to total time to be changed further in the same direction as it was changed by the temperature deviation. An additional corrective effect in the rate of heat application is thereby produced, the magnitude of this effect being dependent upon the extent of the original temperature deviation, and upon the rate v of reset at which the apparatus is set to operate.

Accordingly, the above described operation will be that required to adjust the average rate of heat input to the furnace as necessary to minimize the magnitude of the temperature excursion, and to return the temperature exactly to the control point value with a minimum of over-correction. Once the furnace temperature has been so returned to the control point value, the cycling operation of the relay and furnace heater will continue at the new rate in the presence of the established reset effect until a subsequent temperature deviation occurs.

The various features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages, and specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 illustrates in modified block diagram form a preferred embodiment of the control apparatus of the present invention in combination with a furnace whose temperature is to be controlled;

' Fig. 2 is a circuit diagram illustrating in more Thus, the aver- Fig. 4 is a circuit diagram illustrating in still more detailthe apparatus of Fig. 1;

Fig. 5 is a circuit diagram illustrating in detail the amplifiers of Fig. 4; and

Fig. 6 is a view, taken partly in section, of one of the terminal devices of Figs. 2 and 4.

Description of the Fig. 1 apparatus In Fig. l, we have illustrated in modified block diagram form a preferred embodiment of the time-proportioning controlapparatus of the present invention in combination with a furnace, the temperature of which is to be controlled by the apparatus so as to be maintained at a predetermined, desired value. This showing ofan electric furnace as the device controlled by the apparatus of the present invention is made by way of example, the control apparatus being well adapted to control widely different devices or processes also, such as the wide car bottom furnaoe mentioned hereinbefore, rotary kilns, and other arrangements for use with which time proportion ng control is desirable if not requisite.

The electric furnace of the Fig. 1 arrangement is designated at I, and contains a heating element or heater 2 of the usual form for heating the furnace in the conventional .manner. Within the furnace I is also a thermocouple 3 which is adapted to respond to the furnace temperature. The thermocouple 3 is connected by conductors 4 and 5 to the input of a measuring instrument 5, shown as being of the circular scale type and having a temperature indicator 1 which is power-positioned by the instrument 6 so as to indicate by its position the temperature within the furnace I as measured by the thermocouple 3.

The control apparatus portion of the Fig. 1 arrangement includes a control or instrument slidewire 8, a control or bridge circuit 9, a cycling control amplifier ll} including control relays, and a reset amplifier ll. Also shown in Fig. 1 are a fast reset rate control i2 and a cycle time control l3.

As shown in Fig. 1, the instrument slidewire 8 includes a movable contact 14 which is mechanically coupled by a linkage [5 to the indicator 1 of the instrument 6 in such a manner that the contact [4 is positioned along the slidewire 8 in accordance with the furnace temperature as measured by the instrument 6. Throughout .the following description, the predetermined value at which the control apparatus is intended to maintain the furnace temperature will be referred to as the control point temperature, and it will be assumed that, when the furnace temperature is at the control point value, the instrument 6 will position the contact It at the mid-point of the instrument slidewire 8. Accordingly, the contact i4 is shown in Fig. 1 as being in the control point position, or simply at the control point.

As those skilled in the art will understand, the particular measured furnace temperature at which the instrument slidewire contact M assumes the control point position can be adjusted in the known manner by adjustment of ,the mechanical linkage I 5 which connects the instrument indicator 1 with the contact I4.

The slidewire 8 is connected to the bridge circuit 9 which in turn is operative to supply proper control signals to the inputs of the cycling conaceasse Q trol. and reset. amplifiers I0. and II. Each of'the l'attersupplies control signals to the bridge circuit 5, all as shown in.Fig..1. Thefast. reset rate control I2 isconnected internally to the reset amplifier II,. while the cycle time control I3 is connected between the output andhinputi of the amplifier, III. Theiurnace heater 2 is connected to energizing current supply lines. I55 and I1 through the relays-included in the-am plifierI-O.

Operation of the Fig; 1 apparatus In describing the operation of the time-proportioning control apparatus of Fig. 1, let it be assumed that the temperature of the furnace I is at the control point value, wherebythe con-- tact I4 will be positioned at the control point. Under this condition, there will be. no control signal applied to the reset amplifier II from the bridge circuit 9, but the latter will supply a con.-. trol signal to the input of the control amplifier III in. accordance with the. characteristics of the: cycling and slow reset. control signal fedloackz from the amplifier Ill, whereby the flow or e'nere gizing. current to the furnace heater *2 will. .be. initiated and interrupted alternately in a. mane. ner tending to maintain the furnace temperature constant at the control, point value.

To this end, the control signal applied to the input of the amplifier II] will cause cyclic operation of the control relay of the amplifier at a predetermined rate, this rate being determined by the setting. of the cycle time control I3 in a manner to be described in detail hereinafter. During a portion of each cycle of operation of the amplifiercontrol relay, the relay will be energized or in the. onf position, and hence. will supply energizing. currentto the heater .2 and will. cause heat to be. supplied to the furnace I. at themaximum rate. .The remainder of each of. these operatingcycles will consist of a portion inwhich the .relay is deenergizedor in the off position, during which portion. .the' energizing current to the furnace heater will be turned off and no heat will be supplied tothe furnace I. If desired, the apparatus can be arranged to supply heat to the furnace at a minimum rate when the control relay is in the off position instead of supplying no heat at all as. in the illustrated arrangement.

Under the operating. condition being. described, inlwhich the controlled furnace temperature is at'the control point value, the ratio. of the relay and heater current on time to total time may be such as to cause heat to be supplied to the furnace I at an average rate which is, sufficient to cause the furnace'temperature to be maintained constant at'the" control point value. .If this condition prevails, theslidewire contact'lt wrube maintained at thecontrol point, and the control relay of theamplifierlfl will continue'to cycle with the required ratio or heater on time to total time. Upon the occurrence ofa deviation in'the measured furnace temperature from the control point value, the instrument 6 will cause the contact Hi to be displaced a proportional amount from the control point in a direction corresponding to that of the temperature deviation. If it be assumed that a temperature decrease occurs, let it also be; assumed that the contact I4 is moved to the right'along the slide wire 8 of Fig. 1. Such a furnacev temperature decrease or drop might be caused by an increase in'furnace load, a decrease inthe energizing volt-1 age"between the supply conductors LIB andIT; or'by'any one of other; well-known occurrences.

When the furnace-temperature decreases. and causes movement of the slidewirecontact I4 to the right in Fig; 1, the bridge circuit 9 will change the input signal to the amplifier II] as necessary to increase the ratio of the length of the portion of each operating cycle in which energizing current is supplied to the furnace heater 2 to the total length of the cycle. In other words; a. control signal will be applied from. the bridge, 9 to the amplifier In which will increase the percent on time or ratio of the heater on time to total time. Neglecting momentarily the rate action effected by the apparatus, which action will be described in detail hereinafter, the percent on time will. be increased by an amount which is substantially proportional to the extent. of displacement of the contact It and hence proportional to the decrease in furnace temperature and to the increase of average heat input rate to the furnace needed to return the furnace. temperature to the control pointvalue.

Reset At. this point, it appears to be desirable to define the terms which will be employed consistently herein in describing. the. various phases, operation, and states of the phenomenon known as. reset. as it is produced and functions in the. pres,- ent apparatus. In the following discussion, the terms reset or reset effect will, refer to the conditions established in the apparatus, which cause different rates of furnace heat input. to be maintained under different furnace load or an: alogous conditions as necessary to hold, the con-. trolled temperature at the control point value. The terms reset action or changes in reset will refer to changes in the effective magnitude of the reset, or simply to. changes in the reset effect, while the terms rate of reset. or reset rate" will refer to the rate at which the reset action takes. place, or simply the rate at which changes in the reset effect are produced.

As will be explained more fully hereinafter, the present apparatus is operative to provide two types of control of the bridge circuit 9 which result in the production of reset effects in the operation of the apparatus. Specifically, under appropriate conditions .to be discussed belciv, the reset amplifier II is operative by means of the fast reset control signal to cause the bridge cir-i cuit 9 to effect a reset action,'and hence to change the magnitude of the reset effect present in the apparatus, at a controllable, relatively fast rate; this rate being determined by the setting of the control..l2. The reset effects so produced, and the apparatus components and signals involved in producing these effects, will, for convenience of explanation, be identified, herein by the term fast. Thus, for example, the output signal from the reset amplifier I I has beentermeda fast reset control signal, and the reset effects estab lished by this signalwill be hereinafter referred to as fast reset effects, established by the fast reset action.

In addition to the above, the control amplifier i8 is operative under appropriate conditions'to cause the bridge circuit 9 to'efiect a reset action and to change at a fixed, relatively slow ratethe magnitude of the reset eifect present in the apparatus, this action being accomplished through'the me-' dium of the slow reset control signal. In orderto distinguish this operation from that of the fast reset arrangement? described ffabove; j therresct effects produced under the control Qf'theamplie fier it, and the apparatus components. andsig nals involved in producing these effects, will be identified herein by the term slow. Thus, for example, the reset effects established by the amplifier I will be hereinafter referred to as slow reset effects, these effects being produced by the slow reset control signal. It is to be understood, however, that once reset effects are established in the apparatus, they exercise exactly the same effect on the control functions of the apparatus, no matter whether they are produced at a fast rate by the fast reset portion of the apparatus or at a slow rate by the slow reset portion thereof.

Continuing the description of the operation of the subject control apparatus, the bridge circuit 9 will apply an appropriate control signal to the input of the reset amplifier I! as soon as the instrument slidewire contact If is displaced from the control point. The amplifier in turn will then feed back to the bridge circuit 9 a fast reset control signal which will have the effect of more or less gradually increasing to an even greater extent the relay ontotal time ratio, and which will, therefore, increase further the rate of heat application to the furnace I. In other words, the fast reset control signal applied to the bridge circuit 9 will modify the input signal sent to the amplifier I0 so as to increase gradually the relay on-total time ratio and the average rate of furnace heat input above the new, higher values to which these quantities were quickly raised by the change in the control signal produced by the displacement of the contact It. It will be noted that the effect of the operation. of the reset amplifier II is to control additionally the amplifier H3 in a gradual manner and in the same sense as the control effected by the slidewire contact.

A change in the relay on-total time ratio also produces the aforementioned slow reset action by means of which the magnitude ofrthe reset effect already established in the apparatus by the fast reset action is further increased at a relatively slow rate. Accordingly, the fast and slow reset actions cooperate in changing the amount of reset effect present in the apparatus as required to insure minimum departure from, and quick return to, the control point value of the controlled temperature.

Since the theory of automatic reset and the advantages obtained therewith are generally wellknown in the art, no detailed description of the basic concepts thereof appears to be necessary herein. Suffice it to say, therefore, that the application of the fast and slow reset control signals to the bridge circuit 9 causes the fast reset action to effect a relatively rapid change in the average rate of furnace heat input, in the same direction as and in addition to the so-called proportional change, and secondly causes the slow reset action to effect an additional relatively slow change in the average heat input rate in the same direction. Although the fast reset action is terminated upon return of the temperature to the control point value, as will be explained hereinafter, the necessary reset efiects are maintained in the appa: ratus and continue to provide the necessary control of the amplifier I0 so as to permit the rate of heat input to the furnace to be maintained at an increased value even though the furnace temperature has returned precisely to the control point value.

Before completing the discussion of the reset effects produced in the apparatus, it should be noted that the rate at which the amplifier I I controls the fast reset action within the apparatus is determined by the setting of'the fast reset rate control I2. As will be described below, the mag-' nitude of the fast reset controlslgnal supplied to the bridge circuit 9 by the reset amplifier II is substantially proportional to the amount of displacement of the contact I4 from the control point over the normal operating range of this contact. I Summarizing the above, in the operation of the apparatus disclosed in Fig. 1, a load or other analagous change associated with the furnace I- which causes a decrease in the furnace temper'aei ture below the control point value will at once produce proportional movement of the instrumentslidewire contact M from the control point which in turn will quickly proportionally increase the percent of heater energizing current on time, with the result that the rate of heat input to the furnace will be proportionally increased. While the increased rate of furnace heatinput is acting to return the furnace temperature to the control point value, the signal from the reset amplifier I i will be effective to modify the signal applied to the amplifier I8 from th bridge circuit 9 in such manner as to increase the rate of heat input to the furnace even though the temperature therein is increasing toward the control point vallie and the contact I4 is moving back toward the control point. Finally, the operation of the apparatus will stabilize in such a manner that there will be present in the apparatus the required magnitude of reset effect for the purpose of maintaining the increased rate of furnace heat input needed to hold the furnace temperature substantially at the control point value in the face of the increased furnace load or analagous condition.

Description of the Fig. 2 apparatus The manner in which the apparatus of Fig.1 performs the operation described above, and the manner in which the apparatus is constructed so as to carry out its operation, will be described in the following paragraphs in connection with Figs. 2 through 6. Thus, Fig. 2 illustrates the control apparatus of Fig. 1 of the present invention in somewhat more detail than is shown in Fig. 1, especially in connection with the bridge circuit 9. As can be seen from Fig. 2, the bridge circuit 9 includes an energizing branch I8 having therein the secondary winding I9 of a bridgeenergizing transformer 20. The latter also has a primary winding 2| which is connected to, and energized from, the electrical supply conductors I S and I1. For the purposes of the following description, it will be assumed that the condu ce tors I6 and I1 supply to the apparatus alternatmg current of commercial voltage and frequency: namely, volts and 60 C. P. S.

The bridge 9 also comprises an instrument slidewire branch 22, including the slidewire 8, a cycling follow-up branch 23, and a resetbranch 24. Each of branches I8, 22, 23, and 24 is con. nected in parallel with each of the other branches by means of energizing conductors 25 and 26f The cycling follow-up branch 23 includes an off resistor 21 and an on resistor 28 connected in series between the conductors 25 and 26. Similarly, the reset branch 24 includes an on resistor 29 and an off resistor 30 connected in series between the conductors 25 and 26. For a purpose to be noted hereinafter, devices having high thermal capacitance and designated at 3| and 32 are respectively associated w1th the resistors 29 and 30. 1

The secondary winding I9 of the transformer 20 is provided with a center-tap connection'or terminal 33 which. is connected, as shown, to the arrest ground system ofthe apparatus-at 34. .ACCQrd- .-3-3.an.a1ternating voltage signal dependent in magnitude upon, the amount of displacement between the contact [,4 and its mid-point or control .point position, and having a phase which is either the same as, or is displaced substantially. 130 from, the phase of the supplyvoltage between the conductors l6 and l'l;, depending upon the direction of the displacement of the contact Hi from the control point. The phase of the voltage between the conductors iii and 11,1 and of the .voltage between the conductors25 and 25;, will hereinafter be referred to as the supply voltage or reference phase. a

In a similar mannenthe combination oithe cycling follow-up branch 23 and the energizing branch. I8 of the bridge circuit 9 can be conveniently called a follow-up bridge 23 -4 8. The f rst output terminal of this bridge is the junction 35 between the resistors 21 and 28, and the second output terminal of this bridge. is the terminal In like manner, the reset branch 24 and the energizing branch It can be conveniently defined as a reset bridge 24.IB,.having the junction. 3% between the resistors 29 and 33 as its first output terminal, and having the terminal 33 as its second output terminal.

. As will be obvious from a consideration of Fig.

2, the output of .thefollow-up bridge 23-l8 apnearing between the terminals 35 and 33 Will be an alternating current signal having a magnitude dependent upon the difference between the resistance of the resistor 27 and the resistance or" the resistor 23. Thu's,..w.hen the resistances of the resistors 21 and. 28 are equal, the output of the follow-up bridge. will be zero... Eurther, the follow-up Jbridge output signal will have a phase which is either the same as that of the supply voltage .between the conductors l6 and H or is 180 out of phase with said supply .yoltage, de-

pending upon which of resistors. 21 and 28 has the lower resistance value. Similarly, the output of the reset bridge 2 l.-l8 between 'thejterminals 36 and 33 will be an alternating current signal of a magnitude dependent upon the relative resistances of the resistors 29 andiifl, and having a phase dependent upon which of resistors '29 and 38 has the lower resistance value.

For the purpose of permitting the resistances of the resistors 21', 28, 29, and 30 to berelatively heater 3% is thermally associated withth'e on reset resistor 29, while an Olfflreset heater 4% is thermally associated with the iofi reset resistor As noted in the descriptionv relative to. Fig. 1, the cycling control and follow-up amplifier l0 In. addi- Thus, thermally associated and the reset amplifier ll are controlled by signals supplied from the bridge circuit 9. Specifically, as can be seen iromFig. 2, the amplifier I9 is controlled in accordance with the relative magnitudes and phases of four signals: namely, the output signal from the slidewire bridge 22-48, the outputsignal from theiollow-up bridge. 23 l3;, the output signal from the reset bridge -2 l.i 8, anda cycle time control signal supplied through the cycle time control It in a manner to he described hereinafter, In crder to accornplish this, one input terminal 4! I of the amplifier. ill is connected through resistors 42 and to the output terminal Id of the slidew-ire bridge, and is connected through aresistor 44 to the output terminal 35 of the follow-up bridge The input terminall. s1 is also connected through ,a .resistorflii to the output terminal 360i the reset bridge, and to the cycle time control resistor in a manner to be later described. The other input terminal 46 of the amplifier i0 is connected by a grounded conductors? to the terminal33 of the energizing branch l8, whereby the outputs or the slidewire, follow-up, and reset bridges are effectively connected in parallel across the input of .the amplifier ll). Accordingly, the latter is jointly controlled by four signals through the circuits just described.

The reset amplifier, iscontrolled solely in accordance, with the output signal of the slidewire bridge 22-18, and, to this end, one of the input terminals 438 of the amplifier lj islconnected to the terminalor contact M of the slidewire bridge through the resistor ,43-, while the other input terminal 49 of the-amplifier ll is'connected to the energizing branch terminal}; by the grounded conductor 41. Asa result of these connections, the reset amplifier H is controlled in accordance with the magnitude and direction of. displacement of the contact [4 from the control point along the instrument slidewire 8.

. The heating of the cycling follow-up heaters 37 and 3.8 is controlled by thejamplifier l0, and hence by the resultantoi the three output signals of the three bridges constituting the bridge circuit 9, neglecting momentarily the cycle time controlsignal. To this end, the heaters 31 and 3.8 are connected in series between theenergizing conductors 25 and 26, whereby the heaters 31 and .33 would. be substantially equally energized were it not for the contro in a c the amplifier it. As shown, a movable contact member 50 of a relay device M is connected to the ,junction between the heaters 31 and 3-8, while a normally open relay contact 52 and anorrnally closed relay contact 53 are respectively connected to the conductorszt. and 25. Therelay 5l is actually included in theyamplifier ill of Fig. 1, and includes other contacts not shown in Fig. 2. However, the relay 5! has been'shown in Fig. 2 physically removed from theQamplifier l0 so as to make more clearly apparent the actual heater controlling connections between the amplifier I 0, relay 5-!,.and heaters 3'1 and 3 81. The operating (coil or winding 54 of the relay 5! is shown as being connected by conductors .55 and 56 to the-:out-

put terminals .51 and 5.8 oftheamplifier-l9;

1 :A can r a e seen:frQm1-F t er lay onta ts. .56 and; when engaged; effectively short circuit the heater 3! and'thereby j gizethe latter. similarly the relay contacts 50 and 5-3, when engaged, efiectively shortcircuit and deenergize the;heater-3B.j- As noted "above, the contaqts 5i and 52-arenormally out of engagement or open, which meet. sathattheseeomreferred to as an off terminal.

facts are closed only when the coil or winding 54 of the relay 5| is energized. The connections between the winding 54 and amplifier l are such that the relay is deenergized when the resultant voltage applied to the amplifier input terminals 4| and 46 is zero. Accordingly, when an input signal of zero magnitude is applied to the amplifier input terminals 4| and 46, the relay 5| will be deenergized, no energizing current will be supplied to the heater 38, and full energizing current will be supplied to the heater 3'1. Since, as will be explained hereinafter, the load consignal applied to the amplifier l0, and the relay and controlled load are in the off condition.

When a signal of requisite magnitude and of the phase of the supply voltage is applied to the input terminals 4| and 46 of the amplifier ID, the relay 5| becomes energized, whereby the contacts 50 and 53 are opened and the contacts 56 and 52 are closed. This causes the complete deenergization of the heater 3'! and full energization of the heater 38.

Since the load controlled by the apparatus is energized or turned on by the energization of the relay 5| as will be explained below, the energized condition of the relay 5| is referred to as the on condition thereof. This explains why the heater 3B was earlier referred to as an on heater, it being obvious from Fig. 2 that the heater 38 is energized when the relay 5| is in the energized or on condition.

Although the apparatus of Fig. 2 may be I utilized to control a simple load in a direct on-off manner as the furnace heater 2 of Fig. 1 is controlled, we have illustrated in Fig. 2, by way of example, a more sophisticated form of control for the furnace heater 2. shown associated with the heater 2 a contactor device 59 which is operative to cause heat to be supplied to the furnace at a high rate when the relay 5| is energized and a high rate of heat input to the furnace is needed, and to cause the heater 2 to supply heat at a minimum rate when the relay 5| deenergized. Such an arrangement has been found to be necessary, or at least desirable, in numerous heat controlling applications, as noted above.

Since it is assumed that the amplifier ID of Fig.

' 2 includes the relay contacts which control the load, such as the furnace heater 2, the supply or energizing conductors I6 and I1 have been shown as being connected to energizing terminals 60 and 6| of the amplifier Ill. The latter also has output terminals 62, 63, and 64 which are adapted to be connected to the load to be controlled. The terminal 62 is directly connected within the amplifier III to terminal 6|, while terminal 63 becomes connected to terminal 60 when the relay 5| is deenergized. Accordingly, terminal 63 is When the relay 5| is energized, the terminal 66 is connected within the amplifier ID to the terminal 64 in lieu of the terminal 63. For this reason, the terminal 64 is referred to as an on terminal. Terminals 63 and 64 may also be referred to as cool and heat terminals, respectively. Since the terminal 62 is actually a continuation of supply conductor this terminal is referred to as the common" terminal.

Thus, in Fig. 2, there is Conductors 65, 66, and 61 are shown as being connected to the respective terminals 62, 63, and 64 for connecting the amplifier relay to the controlled load through the contactor 59. If a imple load of the type represented by the furnace heator 2 were to be controlled in an on-off manner as is contemplated in Fig. 1, this load would be connected between the conductors 65 and 61. For loads of the type shown in Fig. 2 which require a. minimum application of heat when the relay 5| is in the off position, the terminal 63 and conductor 66 are utilized. Thus, as shown in Fig. 2, the conductors 65, 66, and 61 are connected to the input or control terminals of the contactor 59, while the output or load terminals of the latter are connected by conductors 68 and 68 to the furnace heater 2. Energizing current for the heater 2 is supplied to the contactor 59 by the supply conductors 69. The latter are connected to a conventional three-wire energizing current supply system which is assumed herein to supply -230 volts to the apparatus in the well-known manner.

The contactor 59 is of the well-known and widely used form employed to connect a threewire supply line to a two-wire load in such a manner that, in one operating condition of the contactor, the load will be energized at the lower of the two voltages of the three-wire supply. When in its second operating condition, the contactor 59 is adapted to energize theload with the higher of the two voltages of the three-wire supply. In Fig. 2, the operation of the relay 5| controls the contactor 59 and causes it to assume one or the other of its two operating conditions by virtue of the connection of theconductors 65, 66, and 61 between the amplifier l0 and the contactor 59. i

Specifically, when the relay 5| is deenergized or in the off position, the contacts of this relay, included in the amplifier I0, cause the contactor 59 to energize the heater 2 through the conductors 68 and 66 with energizing current at a voltage of 115 volts, this voltage being the lower of the three-wire supply voltages as assumed above. Accordingly, under this condition, heat is supplied to the furnace I at a minimum rate as is-desired.

When the relay 5| is energized or in the on position, however, the contactor causes the furnace heater 2 to be energized with a voltage of 230 volts, this being the higher of the two supply voltages. For this condition of operation, heat is supplied to the furnace l at a maximum rate. Accordingly, operation of the relay 5| between its off and on positions controls the contactor 59 as necessary to cause heat to be supplied to the furnace at respectively minimum and maximum instantaneous rates.

The energization of the reset heaters 39 and 46 is controlled by the reset amplifier II. To this end, the latter is provided with output terminals 10,1I, 12 and 13. The terminals 10 and H are connected by respective partially shown 0011'.- ductors l4 and 15 to the terminals of the off reset heater 40, while the terminals 12 and 13 of the amplifier II are connected by respective partially shown conductors I6 and T1 to the on reset heater 39. The amplifier is so arranged, as will be brought out in detail hereinafter, that the heaters 39 and 40 will be substantially equally energized when no input signal is applied to the amplifier input terminals 48 and 49, asoccurs when the contact I4 is at the control point. For an input signal to the amplifier H of resees,

13 duisite magnitfide ahd of the same phase as that of th supply voltage, the on reset heater 39 will be energi'zed'to a greater eirte'nt than will the oil reset heater 49.- Such an input signal would result from upward movement of the instrument slidewire contact I from the control point, such upward movement being caused by adecrease in me measures furnace temperature below the control point value. Conversely, an increase above the control'point value of the m s es temperature" causes downward movement of the contact 14 in Fig. 2 and results in the ap plicatio n to the-input terminals 43 and t9 of the reset amplifier H of a signal having a phase opposite to that of the supply voltage phase. This in causes the off reset heater 4% to be energized to a greater extent than is the on reset heater 39. In each case,' the magnitude time difference in energization of the two reset heaters 39 and 49 will be proportional to the amount of displacement of the contact It from thecontrol point over thenormal range ofop- "eration of the contact I l.

As previously noted in connection with Fig. l, "the fast reset rate control l2 or the reset amplifier H provides a means for regulating the rate at which reset is effected in theapparatus by the "fastreset action. Electrically, the control l2 permits the adjustment of the amount of differential energization of the reset heaters 39 and 40 for a given magnitude or" input signal applied between the terminals 48 and 9. The cjntrp 12 is provided with a suitable knob is by means o f'vvhich manual adjustment of the reset'rate may be efiec ted.

ns will be described in detail below in connec- "tion 'with the detailed circuit showing of Fig. the reset amplifier l'l' receives energizing power fromthe amplifier l8. For this purpose, a cable "l 9'is shown in Fig. 2 connected'between the am- .J

plifiers Wand H.

Operation of the Fig. 2 apparatus The specific manner in which the apparatus of Fig. 2 operates to effect the controlling actions broadly described in connection with Fig. 1 will now be described. For purposes of explanation, let it be assumed that conditions are such that the instrument slidewire contact i4 is at the control point, which condition is indicative of the fact that the furnace temperature is at the desired value. Under this condition, no input signal will be applied to the amplifier it from "the slidewire bridge 22-IB, since the contact :or terminal 14 will be at the same potential as the terminal 33. Also, no input signal will be applied to the input or the amplifier II for the same reason. With no input signal applied to the amplifier II, the reset heaters 39 and All Will be equally energized. Let it also be assumed that this condition has prevailed for an appreciable length of time, and that the reset heaters-39 and 40 and the reset resistors 29 and 30 are all at substantially the same temperature.

This implies thatthe reset resistors 29 and 30 are 'being heated equally also by the-follow-up heaters 31 and 38, which condition exists solely when the apparatus is operating with a 50% on time and hence with equal-average energization of the -heaters Hand 38. From another viewpoint, the condition of equal temperatures "or the reset "resistors 29 and 30 implies the absence of any fast or slow reset actions or eliects in the apparatus at the time.

"Since the resistors 29 amen can 'be' made to see have substantiallye'qual resistances at a given temperature, and to have substantially equal temperature coeflicients of resistance, the main tenance of the resistors 29 and 30 at the same temperature will cause the reset bridge 24-! to apply no input signal to the amplifier 10, since the terminal .36 will be in the same potential as the terminal 33.

Cycling action Although neither the slidewire bridge nor the reset bridge will be supplying an input signal to the amplifier 2 for the assumed conditions-being discussed, an input signal to the amplifier it will be provided by the follow-up bridge 23-). This signal will be a cycling one, due to the cycling operation of the follow-up bridge and relay 5i. In explaining this cycling action, let the analysis be sta ted at a time when the follow-up heaters 3i and 38 have so heated the respective follow-up resistors 23 and 2:3 as to cause these resistors to have equal resistance values. Since these resistors can be selected to have substantially the same resistances at a given temperature and to have the same temperature coefficients of resistance, the above assumed. condition of equal resistance for the resistors 2? and it implies that these resistors will be at substantially the same temperature. Forthis condition, the terminal 35 will be at same potential as the terminal 33, and no input signal to the amplifier iii will be applied from the iollow up bridge. Consequently, no resultant si nal will be applied to the input of the amplifier ill at this instant, whereby the relay 5! will be deenergized as noted above.

For simplicity, the eilects of the circuit including the cycle time control resistor 13 will not be considered at the present'time. However, the operation of thecycle time control arrangement. will be described in detail hereinafter.

As long as the relay 5% is deenergized, the oil follow-up heater 3'! will be energized, andthe on follow-up heater 38 will be deenergized.

Accordingly, the temperature of the resistor 2! will quickly increase above that of the resistor whereby a potential difference will appear between the terminals 35 and 33. When this occurs, the relay 5! will be caused tov assume its energized position, since an increase in the temperature of the resistor 21 above that of the resistor 23 will cause the resistance of the resistor 2? to decrease, thereby supplying to the amplifier it an input signal having the phase of the supply voltage. As soon as it is energized, the relay 5! will cause the closing of the contacts 5s and 52 and the opening of the contacts be and 53, thereby deen'ergizing the off heater 3? and energizing the on heater 3%.

This reversal in the energ'ization of the-hea ers 3i and 38 will cause the resistance of the on resistor '28 to decrease below the resistance of the off resistor 2i, whereby the potential differenoe between the terminals 35 and 33 will-"be decreased. Consequently, the input signal. to the amplifier 59 will be decreased, and may actually pass through zero and become 183 out of phase with the supply voltage. In any event, the reduction of the magnitude of the input sig nal below a predetermined minimum value will cause deenergization of the relay 5i, thereby again reversing the heating relationships of the heaters 37 and 38. Thereafter, the off theater i will lower the resistance "of the ofi? resistor aeeasse 21, and the cycle will be repeated. In each case, the cooling of the unheated resistor contributes to the establishment of resistance and temperature diiferentials between the resistors 2! and 2B in the same manner as does the heating of the heated resistor.

Accordingly, it can be seen from the above description that the conjoint operation of the heaters 3! and 38, resistors 21' and 28, relay 5|, and amplifier produces a cycling action of the apparatus which causes the relay 5i alternately to assume its energized and deenergized conditions. For the condition of no input signals to the amplifier ID from the slidewire and reset bridges, the relay on time will be substantially equal to the relay off time, whereby the apparatus may be said to be operating at 50% on" time, as assumed above. Under this condition, of course, heat will be supplied to the furnace at the maximum rate for 50% of each cycle of operation of the apparatus. Since the heaters 31 and 38 will then supply heat at equal average rates, the heating actions of these heaters will not eiTect the temperature equilibrium condition of the reset resistors 29 and 3%], and no reset actions or effects will be produced as assumed above.

Cycle time control The operation of the circuit including the cycle time control resistor I3 will now be described. If this circuit were omitted from the apparatus, the latter would cycle under the conditions described above at a rate determined by the thermal and electrical constants of the apparatus, and there would be no convenient means for controlling this rate of cycling. However, since it has been found desirable in many cases to be able to adjust the cycling rate or cycle time of such apparatus, the resistor i3 is advantageously employed in the present invention.

Specifically, the resistor 13 is connected in Fig. 2 between the movable relay contact 5% and ground, and hence to the conductor 4? and amplifier input terminal 46. The movable contact of the resistor I3 is connected through a re sistor 80 to the other input terminal 41 of the amplifier ID. The contact of the resistor 13 is provided with a suitable knob 8| to permit the manual adjustment of the contact along the resistor I3.

From the connections just described, it can readily be seen that an adjustable portion of the voltage appearing between the relay contact 59 and the terminal 33 will be applied to the input of the amplifier :c, the magnitude of this portion being dependent upon the position of the contact on the resistor i3. Since the relay contact 50 is connected to the conductor 25 when the relay 51 is deenergized, and is connected to the conductor 25 when the relay 5! is energized, it is obvious that the signal between the contact 59 and the terminal 33 will be of supply voltage phase when the relay 5! is energized, and will be 180 out of phase with the supply voltage when the relay 5| is deenergized.

thereof, whereby a signal of maximum magni,

tude is supplied to the amplifier from the resistor l3, it will be seen that, when once energized, the relay 5| will tend to remain in this condition for an extended period of time, notwithstanding the tendency of the heated on re sistor 28 and the cooling off resistor 21 to lower the magnitude of the amplifier inputsignal. In other words, the signal of supply voltage phase applied to the amplifier input from the relay contact 5|! when the relay 5| is energized is of such a magnitude as to require the resistance of the on resistor 28 to decrease, and the resistance of the resistor 21 to increase, to much greater extents in order to deenergize the relay 5| than would be required if the signal from the contact were not present. Since it requires a substantial amount of time for. the resistors 21 and 28 to so change in resistance, the application of the signal from the relay contact 50 through the cycle time control connectioninav terially slows down the cycling of the apparatus, or, in other words, increases the cycle time thereof. Accordingly, the uppermost position o.f the contact along the cycle time control resistor I3 of Fig. 2 produces operation of the apparatus with a maximum or slow cycle time.

Conversely, when the contact of resistor i3 is at the lower end thereof,.no signal from the relay contact 50 is applied to the input of the amplifier It. Under this condition, the apparatus operates as though the resistor i3 and its con.- nections were entirely absent, and cycling occurs at the fastest possible rate.

Returning to the description of the operation of the apparatus under the conditions assumed above, and assuming that the cycle time control is is set for the fastest cycle time, the described equal on-oil cycle periods may cause heat to be supplied to the furnace at the rate required to maintain the furnace temperature at the control point value. If this is the case, the apparatus will continue to operate as above discussed, and there will be no need for the reset effects or actions of the apparatus. Such operation will continue in the absence of furnace load or supply voltage changes or other occurrences which would have the effect of disturbing the equilibrium of the apparatus.

The equivalent circuit of Fig. 3

The circuit of Fig. 3 is thought to be helpful in illustrating the manner in which the output signals from the various bridges cooperate and coact to control the amplifiers I i] and II. In Fig. 3, each of the sources of signal voltage o'f Fig. 2 is shown diagrammatically as an adjustable alternating current generator or signal source capable of delivering alternating current signals of variable amplitude and of one phase or the opposite phase. As in Fig. 2, the amplifier I0 is controlled jointly by the output signals of the follow-up bridge 23-|8, the reset bridge 24-48, the slidewire bridge 22l8, and the slow cycle time control circuit, while the reset amplifier II is controlled solely by the signal from the slidewire bridge. To this end, the source 22--l8 of Fig. 3, representative of the output of the slidewire bridge which appears between the terminals l4 and 33, is shown connected directly between the input terminals 48 and 49 of the reset amplifier H. The source 22-48 is shown as being controlled by the linkage 15 which in turn is positioned by the associated measuring instrument as illustrated in Fig. 1.

The source 22-I8 of Fig. 3 is also connected in series with the resistor 42 between a conductor 82 and the grounded conductor 41. The conductor 82 is connected directly to the input terminal 4| of the amplifier Ill, while the input ter- 17 minal 46 of the latter is connected to the conductor A l as in Fig. 2. Y

The voltage source 2 ll8, representative of the output of the reset bridge appearing between the terminals 3.5 and is shown as -being connected in series with the resistor is between the conductors and ill. Similarly, the voltage source 2S-l'e, representative of the output of the follow-up bridge appearing between. the terminals and is shown as being connected in series with the resistor 4 between the conduc tors 82 and 4?. Finally, the source 5b33, rep resentative of the signal appearing between the relay contact 56 and the'terminal 33, is shown as being connected in series with the resistor 86 between the conductors 82 and ll. Fig. 3 makes clearly apparent, therefore, that the four sources 22-!8, 23l$, 21l-l3, and Elk-33 are all connested in parallel across the input of the amplifier iii, each source having connected in series therewith its associated resistor.

Employing the circuit of Fig. 3 in summarizing the operation of the Fig. 2 apparatus as hereinbefors described, it can readily be seen that when the output from the slidewire source 22-! is zero (controlled. temperature at the control point value), there will be no signal applied to the input of amplifier H, and the latter, there fore, will not cause the reset source i k-l3 to supply any reset signals between the conductors 82 and ll or between the input terminals ll and 8 of the amplifier It. Similarly, the source 22-48 will not produce any signal between the input terminals ll and 45. Also, since it was assumed that the cycle time control it was set for maximum cycling rate, the source 56-33 will not produce a signal between the terminals ll and 46.

However, as explained above, the cycling source 23|8 will supply a cycling signal between the amplifier terminals 4! and 4% which in turn will control the source 23-46 by means of the thermal connection 83. This connection is iepresentative of the thermal relationship existing between the follow-up heaters 3'! and 38 and the corresponding resistors Zl and 253 of Fig. 2.

As Fig. 3 makes readily apparent, therefore, the source 23-48 and the amplifier in will continue to cooperate to cycle the load on and off with a 50% on time as long as the resulting rate of application of heat to the furnace is that required to maintain the output of the source 22-!3, and hence of the source 24-43, equal'to zero. If it is desired to change the cycle time in order to lengthen the duration of the individual cycles, the sourc 5b33 may be adjusted by means or" the knob 8| so as to apply to the amplifier iii a signal of suitable magnitude to slow down the cycle time of the apparatus.

Continuing the description of the operation of the apparatus of the present invention, let it be assumed that a change in the furnace load occurs such that the rate of furnace heat input corresponding to 50% on time is insufficient to maintain the furnace temperature at the control point value. For the time being, the operation of the reset amplifier ll and the fast reset action which it produces will not be considered nor described, in order that the slow reset action produced by th cooperation of the reset resistors 29 and 3% the follow-up heater 3! and 38 can be illustrated and described to best advantage.

As a result of the-load and measured furnace temperature changes which were mentioned immediat'ely above, the contact M of Fig. 2 will be moved from the control point in an upward direction corresponding to a measured temperature decrease. This in turn will cause the slidewir' bridge 22-48 to apply a signal of supply voltage phase to the input of the amplifier Ill which will have the effect of opposing the signal from-the follow-up bridge tending to turn the load off and of increasing the ratio of the on time to the total time to some value other than unity. The relationship between these signals and their joint effect in controlling the amplifier Hi can readily be seen from the simplified showing-of Fig. 3, wherein the various signals will be assumed to have the relative instantaneous polarities indicated.

Were there no reset actions whatsoever in the apparatus being discussed, the operation just described would continue until th measured furnace temperature'started to increase in a direction to return to the control point value, whereby the signal from the slidewire bridge would commence to decrease. An equilibrium condition would later be established wherein the contact l4 would occupy a position displaced from the control point, and the furnace temperature would be maintained somewhat below the control point value, as necessary to cause the sli-de wire bridge to impress on the amplifier I!) a sufiiciently large signal to maintain the percent on time high enough to cause the average rate of heat'input to the furnace to be that necessary to maintain the last mentioned temperature. Such operation would be undesirable in most cases, however, since it would result in a furnace temperature offset 01' droop, the furnace temperature being maintained at some value other than the intended control point value. Because of the presence of the reset bridge in the apparatus being discussed, however, the droop in'thecontrol'temperature just described is prevented from occurring as will now be explained.

The slow reset action produced in the apparatus by the action of the follow-up heaters 31 and 38 on the reset resistors 29 and 30 will be described below in connection with Figs. 2, 3, and 4. The latter is a circuit diagram of the apparatus of Fig. 1 which is similar to th circuit diagram of Fig. 2 but illustrates the apparatus in somewhat more detail than does Fig. 2. Specifically,- thecircuit of Fig. a illustrates the manual control portion of the apparatus, to be described hereinafter, and also shows in more detail the thermal relationships existing between the various heaters and resistors of the follow-up and reset bridges. of 'Fig. 2. In each figure, the same component is designated by the same reference character.

Description of the Fig. 4 apparatus As shown. in Fig. 4:, the oil follow-up heater 3?, the off follow-up resistor 2'5, the oil reset resistor 30, the off reset heater 4e, and the high thermal capacitance device 32, associated with the resistor 39 and heater ii), are contained within an cit thermal unit t i, while the on follow-up heater 38, the on follow-up resistor 28, the on reset resistor 28-, the on reset heater 39, and the thermal device 3!, associated with the resistor 29 and heater 39, are contained in an on thermal unit 85. As shown b the double headed arrows in Figs. 2 and l, and as will be explained hereinafter, there is heat coupling between the heater 3'! and the resistor 30 1' as well as the resistor 21, and between the heater #59 and the resistor 21 as well as the resistor 30. Additionally, there is heat coupling between the resistors 2'5 and 39 themselves. However, the heat coupling between the follow-up elements 21 and 3t at the left-hand end of the unit84 and the reset elements 39 and 40 at the right-hand end of the unit 84 is a low conductance coupling, whereas the coupling between the follow-up elements 2'! and 31 themselves is a h gh conductance coupling, as is the coupling between the reset elements 39 and 40.

Similarly, there is heat coupling between the heater 38 and the resistor 29 as well as the resistor 28, as there is between the heater 39 and the resistor 28 a well as the resistor 29. In this case also there is heat coupling directly between the resistors 28 and 29. As is the thermal unit 84, however, the heat coupling between the followup elements 28 and 38 at the left-hand end of th unit 85 and the reset units 29 and 39 at the right-hand end of the unit 85 is a low conductance coupling, while a high conductance coupling exists between the follow-up elements 28 and 38 themselves, and between the reset elements 29 and 39 themselves.

In each of the units 84 and 85, the respective devices 32 and 3| of high thermal capacitance cause the temperatures of the respective reset resistors 38 and 29 and the respective reset heaters 49 and 39 to change more slowly than do the temperatures of the follow-up resistors and heaters at the left hand ends of the units 84 and 85 in Fig. 4.

In Fig. 4, the relay shown separate from the amplifier In in Fig. 2, is assumed to be contained within the block representing the amplifier I0. Accordingly, the amplifier ID in Fig. 4 is connected by conductors 86, 81, and 88 to the follow-up heaters 31 and 38 which, in Fig. 2, are shown directly connected to the contacts of the relay 5|. Specifically, the amplifier ll] of Fig. 4 is provided with heater energizing terminals 89, 99, and 9| which are assumed to be connected within the amplifier to the respective contacts 52, 50, and 53 of the relay 5|. The conductor 86 connects the amplifier terminal 89 to the terminal of the heater 3! which is connected to the conductor 25, while the conductor 81 connects the amplifier terminal 90 to the junction between the heaters 31 and 38. Finally, the conductor 88 connects the amplifier terminal 9| to the terminal of the heater 38 which is also connected to the conductor 26. In Fig. 4 as in Fig. 2, therefore, the heater 3! has connected thereacross the normally open relay contacts 59-52, while the heater 38 has connected thereacross the normally closed relay contacts 50- 53.

Slow reset action As was stated hereinbefore, the action of the apparatus, in the presence of a load or analogous change which causes upward movement of the instrument slidewire contact I4 from the control point in response to a temperature decrease, is to apply to the amplifier I9 an appropriate input signal which increases the on time to total time ratio of the operation of the relay 5|. This results in an increase in the rate of average heat input to the furnace I, as previously noted. In addition to effecting the follow-up action described above, the resulting unequal average heating of the heaters 31 and 38 and resistors 21 and 28 will quickly subject the reset resistors 29 and 39 to an unequal average heating sheet,

and will gradually cause the resistor 29 to as! the llnesses rate of sume a higher temperature than that possessed by the resistor 30. This temperature differential will occur'even though the reset heaters 39 and 49 are equally energized, as it is assumed they are in order to simplify the present description by neglecting the fast reset action produced in the apparatus.

As a result of the very gradual increase in temperature of the reset resistor 29 relative to that of the resistor 38 produced by the increased percentage on time of the follow-up heater 38, the resistance of the resistor 29 will slowly decrease with respect to the resistance of the resistor 30, these actions constituting the slow reset action and taking place slowly because of the thermal inertia supplied by the device 3|. The relative change in the resistances of the resistors 29 and 30 will also be augmented by the reduced average heating of the oil cycling heater 3! and the consequent gradual reduction in temperature and increase in resistance of the reset resistor 39.

The occurrence of a resistance differential between the reset resistors 29 and 39 will obviously result in the production of an output signal by the reset bridge 24|8, and hence in the establishment of a reset effect. Since the assumed operation results in a decrease in the resistance of the resistor 29 and an increase in the resistance of the resistor 30, the resulting increase in the potential between the terminals 36 and 33 will be such as to cause the production of an output signal of supply voltage phase. Consequently, this signal will augment the slidewire bridge output signal, and will oppose the follow-up bridge output signal.

The above signal relationships can be clearly seen from Fig. 3, wherein it is shown that the assumed heating effect produced by the amplifier II] will cause the reset source 24-48 to produce a signal having an instantaneous polarity or phase which is the same as that of the signal from the slidewire source 22-| 8, but which is opposite to the output of the follow-up source 23| 8. In Fig. 3, the heat coupling 83 is the follow-up coupling between the heaters 37 and 38 and the respective resistors 2'! and 28, while the heat coupling between the heaters 31 and 38 and the respective reset resistors 39 and 29 is designated in Fig. 3 at 92, wherein it is shown connecting the amplifier It for control of the reset source 24|8.

Since the signal produced by the reset bridge is opposite in phase to the follow-up signal but is of the same phase as the output signal of the slidewire bridge, the first mentioned signal acts on the amplifier H), in the same manner as does the slidewire signal, to increase to an even greater extent the on time to total time ratio of the operation of the relay 5|. Accordingly, the operation of the reset bridge under the influence of the follow-up heaters augments the effects of the upward displacement of the contact 4 and causes an additional increase to occur in the rate of heat input to the furnace I. Further, because of the thermal inertia associated with the reset resistors 29 and 39, the efiects of the reset bridge output signal are not felt at the instant of displacement of the contact M from the control point, but instead become gradually apparent following such displacement.

Continuing the description of the assumed operation of the apparatus of the present invention, let it next be assumed that the effect of furnace heat input is to cause the contact M to start moving baclrtoward the control point. As thecontact l4 so moves, the output from the slidewirebridge will decrease toward zero, which would cause. a. reduction in the newly acquired higherpercent on time. if it were not for the presence of the above described reset bridge signal. The latter, however, will still be increasing-in magnitude even though the slidewire bridge signal will be decreasing, with the result that the percent on time will not be reduced, but will instead be kept at the new, higher value. As the slidewire contact l4 subsequently closely approaches the control point under the influence of the increased rate of furnace heat input, the output from the reset bridge wil'l start to level out at some average value other than zero, and an equilibrium condition will finally be reached wherein the average reset and follow-up bridge output signals will be of equal magnitude, but of opposite phase, and wherein the contact 54 will have returned to the control point and reduced the output of the slidewire bridge to zero. Since the resistors 44 and 45 are advantageously made to have the same resistance values, the above condition will cause the average input to the amplifier ill to be zero. whereby the apparatus will proceed. to cycle with a percent on time as necessary to maintain zero average input to the amplifier Hi. This percent on time will automatically be that required to maintain the furnace temperature substantially at the control point value, since anydeparture from the latter will cause a movement of the contact I l and the production of a slidewire bridge output signal which in turn will upset the equilibrium condition and cause the establishment of a new operating point at which the ratio of theapparatus on time to total time will be maintained at the value required to hold the furnace temperature at the desired value.

Since the action of the amplifier it and the follow-up heaters 011 the reset resistors in establishing the reset effect above described requires an appreciable amount of time from the occurrence of a change in the percent on time, we have chosen to refer to these effects as slow reset effects as previously noted. It should also be clear from the above description that the slow reset effect is an extended or continued one, the necessary amount of reset effect being maintained in the apparatus by the continued unequal average energization of the follow-up heaters, even though the contact M returns to and is held at the control point.

Fast reset action In connection with the need for a fast reset action in apparatus of the type being discussed, it should be. noted that: the slow reset effects and actions just described are not sufiicient in some cases to maintain the measured temperature at the control point value with the required degree of accuracy or stability. Thus, as was noted above, the slow reset effects do not manifest themselves significantly until an appreciable time after the measured temperature has shifted from the control point value and changed the percent on time by a follow-up action. In other words, the slow reset action permits the controlled temperature to vary substantially ifrorn the control point value for an appreciable length of time before bringing the apparatus back into equi-- librium at a new rate of vfuel; supply. Since. such temperature. excursions are. undesirable .oreven intolerableinsome applications, reset action is needed which is capable of faster operation than that supplied by the slow reset arrangement.

As was mentioned hereinbefore, the reset amplifier H receives an input signal from the slidewire' bridge 22-18 immediately upon a departure of. the. controlled temperature from the desired value. Accordingly, upon such temperature deviatiomrthe amplifier II' is instantaneously operative to effect a change in the relative energizations of the reset heaters 39 and All from the equal energization condition which exists when the contact M is at the control point. Assuming as above, that the apparatus has been in a state of equilibrium for an appreciable time without the need for-any reset, it will be conside-red that the reset and slidewirc bridge outputs are zero at the instant which the controlled temperature starts to decrease due, for example, to a sudden furnace load increase or analogous change. Upon such occurrence, the follow-up bridge and amplifier it will cooperate as before to effect a follow-up control action and increase thepercent on time by an amount commensurate with the amount of temperature decrease. At the same time, the reset amplifier H will cause the reset heater 3 9 to be energized to a greater extent than the heater to, the magnitude of the difierential energization being cornmensurate with the magnitude of the temperature decrease and the corresponding amount of displacement of the contact Hi from the control point.

The unequal energization of the reset heaters 39 and-A6 has the same effect of causing the reset bridge to produce an output signal of supply voltage phase as does the average unequal energization of the follow-up heaters and 2-? described above in connection with the slow reset action. However, the production of the reset bridge-output signal is effected much more rapidly by the reset heaters 3c and iii than it is by the follow-up heaters 38 and 3'5 because of thereset resistors .29 and 35 being in better heat transfer relationship with the respective reset heaters 39 and ill than they are with the respective follow-up heaters 3'8 and 31. In other words, the heat or thermal coupling between the reset heaters and the respective reset resistors has a higher conduction than the heat coupling between the follow-up heaters and the respective reset resistors. Accordingly, the reset bridge produces under. the-action of the amplifier it and heaters 38 and 4.0 a reset signal which is of the same character as that described above inconncction with the slow reset action but which is produced and becomes eiiectivemuch more quickly than does the slow reset signal. Therefore, fast reset, signal is operative to perform its function of increasing the apparatus percent on" timev before the. controlled temperature has had the opportunityto depart very far from the con trol, point value.

In Fig. 3, the heat coupling between the reset amplifier H and the reset source 24-5 8 has been designated by the reference character As Fig. 3 makes clearly apparent, the action of the amplifier .H on the reset source E L-l3 through thev heat coupling 93' is of the same nature as the action of the amplifier ti? on that source through. the heat coupling 92. In each case, the signal produced by the reset source E l-58 is of the same phase of instantaneous polarity as the output of the slid'ewire source 22-43,, and hence augments the last mentioned signal in its efiect of increasing the rate of heat input upon the assumed occurrence of a controlled temperature decrease.

As was mentioned above, the relatively rapid appearance of the fast reset output signal effects appropriate increases in the rate of furnace heat input before the measured temperature has normally had time to depart to any great extent from the control point value. This means that a corrective action is applied by the fast reset action at a time so soon after the initial temperature drop as to prevent any substantial temperature excursion below the desired control point value. The extra increase in heat input rate, produced by the reset effects quickly established by the fast reset action, above the heat input increase effected directly by the displacement of the contact I4 acts to minimize the amount of displacement of this contact, and hence the amount of temperature deviation which can occur before the increased heat input rate takes effect and returns the controlled temperature to the desired value.

In the above description of the fast reset action of the present invention, this action has been spoken of as occurring at its maximum rate. However, it has been found desirable under some operating conditions to operate the apparatus with a reset rate which is somewhat less than the maximum obtainable but which is greater n than the minimum reset rate provided by the slow reset action. In order to obtain intermediate values of reset rate in the present apparatus, the resistor H of the reset amplifier Il may be adjusted manually by means of the knob 18, as previously mentioned. The connections of the resistor l2 to the other components of amplifier l will be described hereinafter in connection with Fig. 5.

To return to the description of the assumed operation of the present apparatus, it was explained above that the fast reset signal produced by the reset bridge 24-l8 quickly causes an additional increase in the rate of furnace heat input in order to hold the temperature deviation to a minimum. Under normal conditions, therefore, the additional corrective effect of the fast reset signal will quickly cause the controlled temperature to stop decreasing and to return toward the desired value. Even as the contact 14 closely approaches the control point, and thereby decreases the slidewire bridge output and reset amplifier input signals towards zero, the thermal inertia of the reset bridge will cause the percent on time of the apparatus to be kept at a sufliciently high value to maintain the desired furnace temperature. When the contact M has returned exactly to the control point, of course, the input to the reset amplifier II will be zero, whereby equal energization of the reset heaters 39 and 40 will once again prevail. When this occurs, the effects of these heaters in producing a reset action will diminish, but by that time the effects of unequal average energization of the follow-up heaters on the respective reset resistors will have increased sufiiciently to surplant the diminishing effects of the reset heaters. Accordingly, the follow-up heaters 37 and 38 will gradually take over the control of the temperature of the respective reset resistors 30 and 29 as the respective reset heaters 40 and 39 relinquish their differential temperature control of the reset resistors. As explained hereinbefore, an equilibrium condition will finally be established wherein thecontrolled temperature will be at the control point value, the contact I4 will be at the control point, the slidewire bridge output and reset amplifier input signals will be zero, and the follow-up bridge and reset bridge output signals will cooperate to provide an average input of zero to the amplifier I0 so that the latter will maintain the proper on time to total time ratio of the operation of the apparatus.

In connection with an explanation of the operation of the follow-up heaters in maintaining the necessary amount of reset effect in the apparatus after the contact [4 and controlled temperature have returned to the control point, at which time all reset actions have been terminated by the reestablishment of equal energizetion of the reset heaters and by the establishment of a desired percent on time and a con stant average energization differential for the follow-up heaters, it has been found to be helpful to consider the heat flows and heat losses involved. Thus, starting from an assumed condition of equal energization of the reset heaters, equal average energization of the follow-up heaters, and equal temperatures for the reset resistors and units 84 and 85 (no reset effect present in the apparatus), it will be apparent that a controlled temperature decrease from the control point value will cause heat to flow from the on reset heater 39 and from the on followup heater 38 to the on reset resistor 29 and device 3!. This will occur due to the average energization of the heaters 38 and 39 being raised above the values corresponding to the equilibrium temperatures of the units.

Heat will flow at a faster rate to the resistor 29 from the heater 39 than it will from the heater 38, however, due to the higher conductance of the heat coupling between the elements 29 and 39 than between the elements 38 and 29. Accordingly, the resistor 29 will have its temperature raised at a relatively rapid rate by the heat flow from the heater 39, this being the fast reset action, while the lower rate of heat flow from the heater 38 will at most contribute only a small portion of the heat required to raise the temperature of the resistor 29, assuming that the fast reset action is set to operate at a high rate. At this time, therefore, the slow reset action may contribute little to the establishment of the reset effect. The latter will appear somewhat gradually in any event because of the thermal inertia of the device 3!, The heating action will continue until temperature equilibrium is reached within the device 85, with the latter at a new, higher temperature corresponding to the increased energization of the heaters 38 and 39.

The above described establishment of a reset effect is augmented by the cooling of the heaters 31 and 40 which results from the decreased energization thereof accompanying the increased energization of the heaters 38 and 39. This cooling action, and the accompanying cooling of the resistor 30 and device 32, are accompanied by heat flow from the device 84, since the temperature of the latter must be reduced to a new, lower value commensurate with the decreased energization of the heaters 31 and 40 before equilibrium can be attained.

If it is now assumed that the increased rate of furnace heat input provided by the above actions is that required to cause the controlled tem perature to be returned to and held at the control point value, the apparatus energizations and temperatures will level out at appropriate equiaccuses" 31 and 33 to maintain their respective units 8&-

and 85 at the equilibrium temperature values in the face of heat losses from the units, the ac companying equal energization of the reset heaters 39 and 4c not contributing to the maintenance of the necessary temperature difference between the units 84 and Bil.

Since the temperatures of the reset resistors 29 and 3!] will be those of their respective units 85 and 84, there will be no heat fiow required to or from these resistors in orderto maintain the necessary amount of reset effect in the appa ratus. Accordingly, this reset effect will continue to permit operation at the increased percent on time even though the signalsfrom the slidewire bridge to the amplifiers it and I! have disappeared.

Such operation could not be achievedwithout the illustrated thermal relationships, however, since the reset heaters 39 and it alone could not cause the necessary reset effect to be main tained in the apparatus after the controlled temperature had returned to the control point value. Specifically, if the reset heaters and corresponding reset resistcrs were thermally separate from the follow-up heaters and associated components, no reset eifect could be maintained in the apparatus when at the control point, since the equal energization of the reset heaters, could not maintain the necessary temperature difference between the resistors 2s and 3?} in the face of the inherent heat losses which would be encountered.

It is clear from the above, therefore, that the inclusion of the reset elements in the same units as the corresponding follow-up elements provides means for maintaining the required amount of reset effect in the apparatus, no matter whether obtained by the fast or the slow reset action, and also provides means for effecting the slow reset action itself, which action is capable of establishing a required reset eiiect in the apparatus even in the total absence of the fast reset action. This latter is significant when it is noted that the slow reset action will so operate even upon operative failure of the reset amplifier and/ or the reset heaters.

It will be obvious, of course, that controlled temperature departures in the opposite direc-' tion to that considered above will effect the necessary reduction in the average rate of furnace heat input as required to return the controlled temperature value to the control point. Naturally, controlled temperature departures of different magnitudes and occurring at different rates in various processes possessing different lags will produce follow-up and reset effects which occur at appropriate times and rates which may difier somewhat from those described above by way of example. However, by the proper ad justment of the various controls provided in the present apparatus, the latter is capable of proper adjustment so as to effect the desired controlof practically any process or arrangement, notwithstanding the fact that different arrangements may possess widely different characteristics-and require, therefore, widely different controlling actions.

2e 7 Rate action In addition to the above described cycling, slow reset, and fast reset functions of the apparatus of the present invention, the latter also advantageously provides a so-called rate action function, whereby the magnitude of deviations in the controlled temperature are further suppressed. The rate action of the present apparatus functions to cause a given change in the controlled temperature to effect a corrective action, the magnitude of which is a function of the rate of the temperature change as well as of the magnitude thereof. For example, a given decrease in the controlled temperature below the control point value is made to effect a larger in crease in the percent on time of the apparatus when the temperature drop occurs at a given rate than would be effected if the same temperature drop occurred at a slower rate. Considering the rate action from another angle, it may be said that this action causes a given temperature deviation to provide an initial additional corrective action of a given magnitude which subsequently drops oif to zero, leaving the value of input signal which the apparatus would have provided in the first place if the rate action had not been present.

The rate action occurring in the apparatus of the present invention is produced by thermal time lags associated with the follow-up mechanism. This can best be described by reference to an assumed operating condition wherein, for example, a temperature decrease occurs at a time prior to which the controlled temperature has been at the control point value for an appreciable length of time with no reset action having been necessary. As was explained hereinbefore under the same assumptions, the departure of the controlled temperature from the control point value immediately causes the application of a signal to theamplifier It from the slidewire bridge 22-45, which signal is effective to increase the percent on time of the apparatus by causing the average energization of the on follow-up heater 38 to be greater than the average energization of the off follow-up heater 31. However, the on follow-up resistor 28 cannot immediately assume the new, higher temperature to which the heater 33 is attempting to bring it, since there are certain thermal time lags which must first be overcome. The first of these lags is due to the thermal inertia of the resistor 23 and heater 38 themselves, which inertia must be overcome before the increased energization of the heater 38 can effect an increase in the temperature of the resistor and heater to a new, higher value.

The second effect which retards the heating of the resistor 25; is that produced by the shell or container for the on unit lib. This shell, designated by the reference character 93 in Fig. 4., contributes a certain amount of thermal inertia to the unit, which means that some time is required for the increased energization of the heater st to raise the temperature of the shell. However, the resistor 2s cannot come up to the new temperature until the shell temperature has been brought up to the new value. A second metallic shell surrounding the unit 85 and described below in connection with Fig. 6 also retards the heating of the resistor 23.

Finally, the thermal device ti must have its temperature increased by the heater 38 before the resistor 28 can have its temperature brought up to that of the heater 38. This eifect is produced by the heat coupling between the follow-up elements 23 and 33 at the left-hand of the unit 85, and the reset elements 3|, 29, and 39 at the right-hand end of the unit. Since the thermal inertia of the device 3| is substantial, an appreciable amount of time is required for the last mentioned action to be achieved. Accordingly, the time required to bring the resistor 28 up to the temperature of the heater 38 is increased by this effect also.

In addition to the heating lags associated with the resistor 28 by the various effects just described, there is in addition a cooling lag associated with the off follow-up resistor 21. The decreased average energization of the heater 3'! will cause the temperature thereof and of the resistor 21 to start to decrease below the equilibrium value, but this action will be retarded by several effects which are analagous to the effects which retarded the heating of the resistor 28. The first effect which retards the cooling of the resistor 21' and heater 3] is that produced by the thermal inertias of the resistor and heater themselves. These inertias prevent the immediate cooling of the resistor 21 and heater 3'! to the new, lower temperature commensurate with the decreased energization of the heater 31.

The second cooling retarding action on the resistor 27 is that produced by the shell of the ofi unit 84, which shell has been designated by the reference character 94 in Fig. 4. The shell 94 will have been at some equilibrium temperature while the apparatus was in equilibrium, and will have to have its temperature reduced to the new, lower value before the resistor 2'! can assume its new, lower temperature. A second metallic shell surrounding the unit 84 and described below in connection with Fig. 6 also retards the cooling of the resistor 21.

Finally, the device 32 must be cooled from its equilibrium temperature before the temperature of the resistor 2'! can be effectively decreased to the lower value, this effect being produced by the heat coupling between the left-hand follow-up elements 21 and 3! and the right-hand reset elements 32, 39, and 4B of the unit 84. The high thermal capacity of the device 32 substantially delays the cooling of the resistor 21.

Summarizing the above, a combination of the heating retarding and cooling retarding eifects produced on the respective on and off followup resistors 28 and 2'! gives rise to a rate action effect in the apparatus, since these thermal time lags or delays delay the normal action of the follow-up mechanism and hence delay the production of the opposing follow-up bridge output signal, whereby the slidewire bridge output signal is permitted to effect a greater initial increase in the percent on time than would be effected if the follow-up bridge could act immediately to oppose the slidewire bridge signal. Accordingly, the greater the rate of temperature deviation, the greater the magnitude of this efiect, whereby the term rate action is seen to be proper- 1y applied to the phenomena just described.

In connection with the rate action as described above, it may be well to note that the reset heaters 39 and 40 may, for very fast reset rates, produce a slight decrease in the magnitude of the rate action obtainable. For example, under the assumed conditions above discussed, the increased heating of the reset resistor 39 in the presence of the temperature decrease may reduce somewhat the time required for the device 3|, and hence the resistor 28, to arrive at the new, higher temof the apparatus as described in detail above,

reference to Fig. 3 clearly shows that the slidewire signal 22--I8 is aided in controlling the amplifier II) by the reset signal 24-48, and is opposed by the follow-up signal 23-48. The follow-up signal 23-I 8 is controlled by the amplifier I0 through the follow-up heat coupling 83. Similarly, the reset signal 24I8 is controlled by the reset amplifier I I through the fast reset heat coupling 93, while this amplifier in turn is directly controlled solely by the slidewire signal 22I8. In addition, the reset signal 24--I8 is increased by the slow reset heat coupling 92 from the amplifier III. Finally, the cycle time signal 50-33 opposes the follow-up signal 23-I8 in the control of the amplifier III, and thereby increases the cycle time thereof.

As noted above, the resistance values of the resistors 44 and 45 are advantageously to be equal, while the values of the resistors 42 and at] are chosen according to the voltages supplied by their associated sources in order that the desired resultant voltages will be applied to the amplifier I 0.

Proportional band adjustment As was previously mentioned, the apparatus of the present invention includes a resistor 43, shown in Figs. 2 and 4, connected between the contact I4 and the remainder of the apparatus. The purpose of this resistor is to provide a manual adjustment of the proportional band of the apparatus. Since the subject of proportional band control is one which is well-known in the art, it appears sufficient to state herein that the adjustment of the resistor 43 determines the required magnitude of the deviation of the controlled temperature in either direction from the control point value to cause the apparatus to operate at either maximum or minimum percent on time, depending upon whether the temperature deviation is a decrease or an increase relative to the control point value. Specifically, the magnitude of a temperature decrease which will cause the apparatus to operate with a maximum ratio of controller on time to total time is determined by the adjustment of the proportional band resistor 43. In other words, the magnitude of the controlled temperature decrease or increase which will cause heat to be supplied to the furnace I at the maximum or minimum rates, respectively, is a function of the setting of the resistor 43.

As shown in Figs. 2 and 4, the resistor 43 effects its proportional band width control by being connected in series between the instrument slidewire contact I4 and the inputs to the amplifiers I0 and II. Specifically, the resistor 43 is connected between the contact I4 and the junction between the conductors connected to the input terminal 48 of the amplifier I I and the terminal of the resistor 42 which is not connected to the amplifier I0. The resistor 43 includes a movable shunting contact 96 which is advantageously provided with a knob 91 for manual adjustment msegese 29 purposes. Rotation of the knob .91, therefore, adjusts the width of the proportionalband of the apparatus.

It has been noted under certain operating conditions of the present apparatus that the proportional band width thereof varies somewhat in the presence of changes in the magnitude of the energizing voltage supplied by the conductors Iii and ii, due to the resulting changes in the heating power supplied to the follow-up heaters t1 and 38. In order to minimize this effect in instances where its presence is undesirable, we have found it feasible to form the follow-up heaters 31 and 38 of nickel wire, the positive temperature coefficient of resistance thereof serving to minimize changes in heater energizing power due to supply voltage changes. By this means, the resistances of the heaters 3'! and 38 will vary with the temperatures thereof, whereby supply voltage changes will change both the heater voltages and the heater resistances in the proper manner to cause one effect tocompensate substantially for the other, and hence will not result in significant heater energizing power changes or resulting variations in proportional band width. Any resulting undesirable sensitivity of the heaters to ambient temperature changes can be rendered unobjectionable by proper selection of the resistance values of the resistors 42, 44, and 45 as necessary to cause the temperature sensitivity of the follow-up resistors 21 and 28 to compensate for that of the heaters 31 and 38 in the presence of ambient temperature changes.

Safe failure resistor Also advantageously included in the apparatus, and illustrated in Figs. 2 and 4, is a safe failure resistor 91, which resistor functions to cause the relay to be deenergized, and the controlled load to assume the off condition, upon the occurrence of an interruption in the continuity of the circuit between the slidewire 8 and the components connected thereto by the contact l4. Such an interruption might be caused by dirt interposed between the contact l4 and the resistor 8, by breakage of the lead connected to the contact [4, etc.

As shown, the resistor 91 is connected between the conductor 26 and the junction between the resistor 43 and the conductor connected to the contact l4. Accordingly, upon interruption of the circuit between the contact 14 and the resistor 8, the resistor 43 is essentially connected through the resistor 91 to the conductor 25, whereby signals are applied to the amplifiers H! and H of the same character as would be applied thereto by the normal downward movement of the contact I I in the presence of a sudden furnace temperature increase of large magnitude. Since such a temperature increase would normally cause a quick reduction in the average furnace heat input rate to its minimum value, such an action occurs also, upon failure of the slidewire circuit as just described, through the medium of the resistor 91'.

Manual control Before concluding the description of the arrangernent disclosed in Fig. 4, a description of the manual control provisions of the apparatus will be given. These manual control provisions permit the quantity controlled by the apparatus, the temperature of the furnace I, for example, to be controlled manually ratherthan automatically when conditions arise to make such a change in control feasible.

For manual control purposes, the apparatus of Fig. 4 includes a manual slidewire resistor 98 provided with a movable contact 99 and an as* sociated adjusting knob I00. The slidewire 98 is connected between the conductors 25 and 23 in series with resistors NH and N32. The resistor IOI is connected between the upper end of the slidewire 98 and the conductor 25, While the resistor ['02 is connected between the lower end of the slidewire 98 and the conductor 26. The contact 99 is connected in series with a resistor I03 to a manual contact I04 of a manualautomatic switch. The latter consists of two ganged portions, one of which is shown at 165 in Fig. 4 and the other of which is shown at Hit in Fig. 5 and will be described hereinafter.

The switch portion [05 also includes an automatic contact H3? and a movable contact 108. The latter is connected to the input terminal M of the amplifier l0 and engages the contact I01 when the switch is in the automatic position. The contact N11 is connected to the junction between the resistors 22 and 45, whereby the connections shown in Fig. 2 are completed by the switch portion I when the latter is in the automatic position.

If it is desired to operate the apparatus under manual control, whereby the temperature of the furnace I can be manually adjusted and the automatic control of the furnace temperature can be rendered ineffective, the switch portion I05 is moved to the manual position, in which the contact Hi8 engages the contact HM. This action connects the manual slidewire 98 to the input of the amplifier Ii! in lieu of the instrument slidewire 8, and disconnects from the input of the amplifier ill the output of the reset bridge. Under this condition of operation, the position of the contact 99 along the slidewire 98 will deter mine the percent on time of the apparatus, and hence the average rate of heat input to the furnace I. The follow-up bridge 23!8 will be operative as for automatic control, as will the cycle time control resistor 83. However, the reset bridge, the reset amplifier H, the proportional band control 43, and the instrument slidewire B will have no effect upon the operation of the apparatus or on the rate of furnace heat input or furnace temperature.

Although the output of th reset bridge is disconnected from the input of the amplifier H) when the apparatus is under manual control as described immediately above, whereby the reset means then exert no efiect on the rate of furnace heat input, the reset resistors 29 and 36 will, nevertheless, continue to be heated differentially by the follow-up heaters 3! and 38 in the manner previously describedin connection with the slow reset aspects of the apparatus. Accordingly, an appropriate slow reset signal is pro duced between the terminals 35 and 33 even under manual control operating conditions, though this signal is not then utilized, as noted above.

However, when the apparatus is subsequently returned to the automatic control condition by the manipulation of the switch m5, the reset signal, which has already been established as just noted, is operative to apply its reset effect to the apparatus once, no time being required for the signal and effect to be established. Therefore, if the apparatus has been adjusted in the manual control condition to be controlling at the control point. prior to the act of switching 31 to automatic control, as will usually be the case under normal procedure, the requisite amount of reset efiect will :be instantly available in the apparatus at the time of switching, and the transfer from manual to automatic control will be effected with a minimum of disturbance to the control of the process.

As will be explained hereinafter in connection with a detailed description of the reset amplifier II, the fast reset action is prevented from contributing to the production of the reset signal While the apparatus is in the manual control condition. To this end, the reset heaters 39 and 40 are at that time prevented from differentially heating the reset resistors 29 and 30, whereby the reset signal then produced by these resistors is due solely to the slow heating action of the follow-up heaters, and hence is limited in magnitude to that which will just be required to maintain the necessary percent on time when return is made to the automatic control condition. Consequently, an excessive amount of potential reset effect is prevented from accumulating While the apparatus is under manual control, but the necessary amount of such effect is ready to function when needed.

The control amplifier of Fig. 5

We have illustrated in Fig. 5 the detailed circuit connections of preferred forms of the cycling control amplifier I and the reset amplifier II for use with the apparatus illustrated in Figs. 1 through 4. As shown in 5, the amplifier I0 includes a first amplifying stage triode electron tube I09, a second amplifying stage triode electron tube IIO, and power amplifying triode electron tubes III and H2. Each of the tubes I09 through I I2 includes anode, control grid, cathode, and cathode heater elements in the usual manner. Also included in the amplifier I0 are the control relay BI, a safe-failure relay H3, a neon signal lamp I It, a power transformer H5, and various other circuit components to be described.

As shown, the transformer H5 includes a secondary winding H6, provided with a center-tap connection I I I, and a primary winding I I8 which is connected between the aforementioned energizing or supply voltage conductors I6 and I1. The transformer H5 also includes a low voltage secondary winding H9 which supplies heater energizing current to the tubes I09, I I0, H I, and

H2 through partially shown conductors X and Y. The winding H9 is provided with a centertap connection I which, together with the connection H1, is connected to a ground bus or grounded conductor I2I.

The specific connections through which the transformer secondary winding I I9 energizes the heaters of the tubes I09 through H2 are important from the safe-failure standpoint, as will be brought out hereinafter. Accordingly, these connections will now be described with reference to Fig. 5, wherein each of the tubes I09 and H0 .is seen to have its heater connected across a respective half of the winding H9, while the heaters of the tubes I H and H2 are connected in series across the entire winding II9.

Specifically, one terminal of the heater of the tube I09 is connected by the conductor X to the upper end terminal of the winding H9, while the other terminal of the last mentioned heater is connected by the conductor I2I to the centertap I20 of the winding H9. Similarly, one terminal of the heater of the tube H0 is connected by the conductor Y to the lower end terminal of the winding H9, while the other terminal of the last mentioned heater is connected by the conductor I2I to the center-tap I20 of the winding H9. Accordingly, the heater of the tube I09 is connected across the upper half of the winding H9, while the heater of the tube H0 is connected across the lower half thereof.

The series connection of the heaters of the tubes III and H2 across the winding H9 is achieved :by connecting together one terminal of each of the last mentioned heaters, and by connecting the remaining terminal of the heater of the tube I II to the upper end terminal of the winding H9 through the conductor X. Finally, the remaining terminal of the heater of the tube I I2 is connected by the conductor Y to the lower end terminal of the winding H9.

The input terminal 4| of the amplifier I0 is connected directly to the control grid of the tube I09. The contact of the cycle time control resistor I3 is connected through the resistor to the input terminal II as shown in Figs. 2 and 4, and hence is connected to the control grid of the tube I09. The input terminal 46 of the amplifier I0 is connected to the grounded conductor I2I, whereby the grounded portions of the apparatus are united.

The cathode of the tube I09 is connected through a decoupling and filtering resistor I22 to a terminal I23 which is maintained positive with respect to ground. The tube I09 is thus provided with suitable positive cathode bias or negative control grid bias. A decoupling and filtering condenser I20 is connected between the cathode of the tube I09 and the ground conductor I2I. The resistors 89 and I3 of the cycle time control portion serve as grid resistors for the tube I09 by virtue of their connection between the control grid of the tube I09 and the conductor I2 I. As can readily be seen, the input signal applied to the amplifier I0 is effectively applied between the control grid and cathode of the tube I09.

The anode of the tube M19 is connected through a load resistor i525 and a decoupling and filtering resistor I26 to a terminal I21 which is maintained at a suitable positive potential with respect to ground. A coupling condenser I28 is connected between the anode of the tube I09 and the control grid of the tube H0, this control grid also being connected to the conductor I2I through a grid resistor I29. The cathode of the tube I I0 is also connected to the conductor I2I, whereby the output of the tube I 09 is resistance coupled to the input of the tube H0. A phasing condenser I30 is connected between the anode of the tube I09 and the conductor I2I, and a filtering and decoupling condenser I3I is connected between the last mentioned conductor and the junction between the resistors I25 and I26.

The output of the tube H0 is directly coupled to the parallel connected inputs of the tubes III and H2. is connected directly to the control grids of the tubes II I and II 2. A load or anode resistor I32 is connected between the anode of the tube H0 and the positive potential terminal I21, to which are also connected the cathodes of the tubes III and H2. It is the anode currents of the tubes III and H2 which maintain the terminals I23 and I2! at positive potentials with respect to the ground conductor I2I, and the manner in which this is accomplished Will now be described.

The anode of the tube I I I is connected through the operating winding 94 of the relay 5| to the upper end terminal of the high voltage secondary winding H6 of the transformer H5. A smooth- To this end, the anode of the tube I I0 ergization of the control or load relay 5|.

'33 ing condenser I33 is connected in parallel with the relay winding 54. Also, the anode of the tube II2 is connected through the operating winding I34 of the safe-failure relay II3 to the other end terminal of the secondary winding I I6.

A smoothing condenser I35 is connected in parallel with the relay winding I34. Finally, the terminal I21, connected to the cathodes of the tubes III and II2 is connected through a resistor I36 to the terminal I23, which in turn end terminal of the winding II6 which is connected to the anode of the tube I I I, and by the plus signs shown adjacent the supply conductor I6 and elsewhere in Figs. 2, 4, and 5, the anode voltage supplied to the tube III will be of supply voltage phase, while the anode voltage supplied to the tube II2 will be of a phase which is opposite to the supply voltage phase. The

aforementioned plus signs indicate in the usual manner which terminals, conductors, and points have the same instantaneous relative polarity.

Since the resistors I36 and I31 are effectively included in the combined anode or load circuits of the tubes III and II2, the flow of anode current through these resistors produces across them a direct current potential which is utilized for supplying anode voltage to the other stages of amplification of the apparatus. Specifically, the flow of anode current through the resistors I36 and I31 is responsible for the maintenance of the terminal I21 positive with respect to the conductor I2I, as it is for the maintenance of the terminal I23 positive with respect to the conductor I2I. Since the load resistor I32 is connected to the terminal I21, the anode current flow through the resistors I36 and I31 actually produces the anode supply voltage for the tube IIII. Similarly, the anode supply and grid bias voltages for the tube I39 are derived from the voltage developed across the resistors I36 and I31 by the anode currents of the tubes III and I I2. A filter condenser I38 is connected between the terminal I21 and the conductor I2I, and hence across the resistors I36 and I31, to cause the supply voltage developed across the last mentioned resistors to be satisfactorily free from ripple.

The values of the components of the amplifier I are so selected in the well-known manner that the tubes I II and II2 are caused to conduct current at a minimum rate when no input signal is applied between the amplifier input terminals II and 46. Under this condition, the relays and H3 are maintained in the deenergized state, since the anode currents corresponding to minimum conductivity of the tubes III and II2 are caused to be ineffective to energize the relays. Upon the application to the amplifier input of an input signal ofsupply voltage phase and of a sufilcient magnitude, the conductivity of the tube II I will be increasedsufilciently to cause the flow of the required anode currentto eifect the en- The safe-failure relay II3 will remain deenergized, however, since theinput signal of supply v 1tage phase will-serve onlyto decrease the conductivity and anode current of the tube I I2. This operation is easily explained when it is noted that an input signal of supply voltage phase will cause the control grids of the control tubes III and I I2 to swing in the positive direction relative to their associated cathodes during the half cycles of the Supply voltage in which the anode of the tube III is rendered positive with respect to its associated cathode. This naturally results in an increase in the conductivity of the tube I I I during said half cycles. However, during the alternate half cycles, in which the anode of the tube II2 is rendered positive with respect to its associated cathode, the input signal of supply voltage phase will swing the control grids of the tubes III and I I2 negative with respect to their associated cathodes, whereby the conductivity of the tube I I2 will be decreased below the equilibrium value.

However, upon the application to the amplifier input terminals 4| and 46 of a signal which is 180 out of phase with the supply voltage, which signal will hereinafter be referred to as an out of phase signal, the safe-failure relay II3 will be energized while the control relay 5| will be deenergized. This action occurs by virtue of the fact that the out of phase input signal causes the control grids of the tubes II I and I I2 to be driven positive with respect to their associated cathodes during the supply voltage half cycles in which the anode of the tube II2 is driven positive with respect to its associated cathode.

This action increases the conductivity of the tube IE2 from the equilibrium value, and hence effects the positive energization of the winding I34 of the relay I I3. At the same time, however, the control grid of the tube III is driven in the negative direction relative to its associated cathode during the half cycles in which the anode of the tube I I I is driven positive with respect to said cathode, whereby the conductivity of the tube III is reduced below the equilibrium value and the relay 5| is maintained deenergized.

As its name implies, the safe-failure relay H3 is included in the apparatus to cause the latter to fail safe, or deenergize the controlled load, upon the occurrence of an apparatus defect or component failure of the equipment. To this end, the contacts of the relay II3 are interconnected with the load controlling contacts of the relay 5| in such a manner that various com- I ponents failures within the apparatus will result in the application of heat to the furnace I at the minimum or zero rate, irrespective of what the furnace temperature may be at the time or subsequent thereto. As shown in Fig. 5, and as described above, th supply conductor I1, connected to the amplifier terminal 6|, is connected directly through the amplifier to the terminal 32, to which the common conductor 65 connects the common terminal of the device 59. The amplifier terminal 60, to which the supply conductor I6 is connected, is connected to a movable contact I39 of the relay 5|, the contact I39 cooperating with a normally closed contact I40 and a normally open contact I4I to provide the load controlling actions of the relayIiI. In the present and subsequent discussion, the term normally refers to the deenergized condition of the associated relay, and is employed irrespective of whether the particular relay is energized or deenergized in its normal operation. The normally closed relay contact I40 is directly connected to the amplifier terminal 63 and hence to the low or cool conductor 66. Accordingly, when the relay 5| 

